Post-deflected cathode ray tube



Dec. 20, 1955 H, 5, BLANKs 2,728,021

' POST-DEFLECTED CATHODE RAY TUBE Filed March 18, 1953 wmmm%wmmINVENTOR.

v HENRY S. BLHNKS deflected variety.

' wherein the particularscreen-area' to which the cathodecb-plariargrill mounted in the grates 2,728,622 Patented Dec. 20, W55

POST-DEFLECTED CATHODE RAY TUBE Henry S. Blanks, Princeton, N. 1.,assignor to Radio Cor- This invention relates to improvements incolor-kinescopes and other cathode-ray (CR) tubes of the post- (Apost-deflected CR tube is one ray beam is directed at. any given instantis determined by the deflecting or switching ,yoltages applied to abi-part 'pathof the beamadjacent to the target side of the screen.)

Post-deflected line-screen color-kinescopes (such, for

sent the same or corresponding parts in all figures, l designates atransparent or translucent glass screen plate which is provided on itsrear or target surface with a multiplicity of groups of parallelphosphor strips or lines G (green), R .(red), B (blue) of diiferentcolor-response characteristics. As is conventional, the phosphorcovered" surface of the screen has an electron-permeable conductivecoating 3 thereon which isconnected, as by a lead 5 to a voltage sourceexemplified bythe voltage divider 7. Appropriate operating voltages aremarked on the drawing. A switching grill, mounted adjacent to themetalized target surface 3 of the line screen, is made up of alternateand intermediate wires 9 and 11, respectively, which are connected, atthe sides of the screen (see Fig. 2)

to external leads l0 and 12 respectively. Here the alternate wires 9 areconnected through their common lead 10 to a point on the voltage divider7 several thousand volts negative with respect to the metalized screen3. A source of switching current, exemplified by thesquarewaveoscillator 13, has one terminal connected to the external lead12 for the intermediate grill wires 11 and its other terminal 14 tosubstantially the same point on the voltage divider 7 to which the lead10 for the alternate example, as the one shown in U. S. patent Schroeder2,446,791) operate most efiiciently in television systems of thefield-sequential variety wherein the switching frequency is of the orderof, say, 144 fields per second. If such tubes are-to be used indisplaying NTSC (National Television System Committee) signals usingswitching and keying frequencies of the order of from 3 to 4 megacyclesper second, one can use either sine-wave switching orsquarewave-switching. Sine-wave switching results in a noticeabledecrease in picture-brightness (at least if colorpurity is maintained bybiasing the beam off during transit of the beam from one color to thenext), .and square-wave switching necessitates the use of a costlyhigh-power oscillator (to overcome the high inter-wire andwire-to-screen capacitive effects at the indicated highswitching-frequency). I V

Accordingly, the principal object ofthe present invention is to providea post-deflected cathode-ray tube which canbe keyed and modulated at anydesired speed without loss of brightness and without excessive power.

' Another and related object of the present invention is to provide amethod of and means for minimizing the effective capacitance incolor-screen units of the postdefiected variety, and thus to permitrapid switching with a minimum of power.

The foregoing and otherobjects are achieved in accordance with themethod .of the invention by limiting the application of the deflectingor switching voltages to the particular elements of the switching grillthat'lie within the immediate path of the beam. Thus only a smallfraction of the total capacitance of the screen-unit is charged at anygiven instant and, as a consequence, less charging current is. requiredfrom the oscillator or other source of switching energy.

of the present invention;

Fig. 2 is a partial rear-elevation ot the color-screen unit shown inFig. l; and

Fig. 3 is an enlarged viewof one of the metalized glass intermediate"wires used in the bi-part co-planar switching grill of the color-screenunit shown in Figs. 1 and 2.

In the drawing wherein like reference characters reprewires 9 isconnected. I

In accordance with the invention the wires of the intermediate group 11,have a core 15 (see Fig. 3) constituted of glass or other insulatingmaterial and are coated with a conductive metal layer 17 except forsmall areas 19, 19 near each end of said wires. These otherwise bareglass portions 19, 19' of the intermediate wires 11 are providedpreferably only at their leading end-portions 19 with a filling orbridge 21 consisting either of a photoconductive material (such, forexample, as cadmium sulphide), or of a B. I. C. (bombardment inducedconductor) material (such, as for example magnesium fiuoride) which-when energized, in a manner later described by the scanning beam 23(Fig. l), operates to connect that particular wire to the source 13 ofswitching voltage.

The photoconductive bridge 21 between the metalized parts of each of theglass wires 11 is rendered conductive by light picked-up from the screenwhen it is energized by the electron-beam 23 from a single electron-gun(not shown). The light that renders the photoconductive bridge material21 conductive may be derived either from the color-phosphor lines (G, Ror B) or from an auxiliary of -scan of the beam. The decay time of thephotoconductive material should be not less than the time required forthe scanning of one line and as little longer than this time aspossible.

The direction-of-scan is along the color-phosphor lines G, R and B onthe screen. The scanning beam is preferably keyed on" at the beginningof each line-of-scan so that irrespective of the amplitude of the videosignal at the sides of the picture, the intensity of the beam as itstarts its excursion will 'be sufiicient to render the bridgeconductive. When the bridge or filling 21 on each metalized wire 11comprises a bombardment induced conducting material (instead of aphotoconductive material) the auxiliary phosphor strips 25 on thescreenplate may be omitted.

It-will be observed upon close inspection of Fig. 1 that since thescreen 1-3 is maintained at a potential several thousand volts positivewith respect to the bi-part coplanar switching grill 9ll, acylindrical-lens field is established in the grill-screen space, astaught in Flechsig's French Patent 866,065. As a consequence the beamelectrons that pass between the grill wires are focused sharply upon theparticular color line'to which they are directed by the instantaneousrelative voltage of the alternate and intermediate wires within theimmediate path of the scanning beam.

Cadmium sulphide, cadmium selenide, selenium, thallous sulphide, leadsulphide, zinc oxide and copper oxide are typical of thepho'toconductors which may be used as the bridging material. All ofthese photoconductors may also be classified as bombardment inducedconductors since they are rendered conductive by beam impact, as aremagnesium fluoride, silica, germanium and numerous other B. I. C.materials which may be employed in the practice of the invention.

What is claimed is:

1. Method of operating a post-deflected cathode-ray tube of the kindhaving a beam-deflecting grill made-up of spaced-apart elements betweenwhich electrons from a scanning-beam pass in their transit topre-selected elemental areas on a nearby screen; said method comprisingapplying beam-deflecting voltages only to those elements of said grillthat lie in the immediate path of scan of said beam.

2. In a cathode-ray tube having an electron-gun and a screen mounted ina position to be scanned by a beam of electrons from said gun, abeam-deflecting grill disposed in the path of said beam and having amultiplicity of discrete beam-deflecting elements extending across saidscreen, an external lead for supplying beam-deflecting voltages to saidgrill elements, and means responsive to the instantaneous position ofsaid beam for electrically connecting to said external lead only thosebeam-deflecting elements that lie in the immediate path-of-scan of saidbeam.

3. In a cathode-ray tube having an electron-gun and screen, an externallead for supplying beam-deflecting voltages to said grill elements, andmeans responsive to the instantaneous position of said beam upon saidscreen for electrically connecting to said external lead only thosebeam-deflecting elements that lie adjacent to said instantaneousposition of said beam upon said screen.

4. The invention as set forth in claim 3 and wherein said lastmentionedmeans comprises a photoconductive bridge connecting said beam-deflectingelements to said external lead.

5. The invention as set forth in claim 4 and wherein saidphotoconductive bridge comprises cadmium sulphide.

6. In a cathode-ray tube having an electron-gun and a screen mounted ina position to be scanned by a beam of electrons from said gun, abeam-deflecting grill 'disposed in the path of said beam and having amultiplicity of discrete beam-deflecting elements extending across saidscreen, an external lead for supplying beam-deflecting voltages to saidgrill elements, and means responsive to the instantaneous position ofsaid beam upon said beamdefiecting elements for connecting only thebeam-impacted ones of said elements to said external lead.

7. The invention as set forth in claim 6 and wherein said last-mentionedmeans comprises a bridge constituted of a bombardment induced conductingmaterial connecting said beam-deflecting elements to said external lead.

8. The invention as set forth in claim 7 and wherein the material ofwhich said bridge is formed comprises magnesium fluoride.

References Cited in the file of this patent v UNITED STATES PATENTSHansen Sept. 18, 1951

